1. Field of the Invention
This invention relates to calcium phosphate compositions, including cements and pastes, and to methods for making and using them. In its preferred form, the invention relates to methods for making hydroxyapatite forming cements and pastes without the necessity for employing tetracalcium phosphate as a precursor.
2. Description of Related Art
In the area of dental cements, the prior art includes a number of compounds. Some such cements, however, irritate the pulp and are unsuitable for applications where the cement must come in contact with exposed pulp. Guide to Dental Materials and Devices, 7th Ed. (ADA 1974) p. 49. One solution to this problem is a cement made of materials similar in composition to tooth and bone mineral, since this would not irritate the living tissue. It has been known for some time that hydroxyapatite materials have the basic properties of human bones and teeth.
The use of .beta.-Ca.sub.3 (PO.sub.4).sub.2 was suggested for pulp capping in Driskell et al., "Development of Ceramic and Ceramic Composite Devices for Maxillofacial Application", J. Biomed. Mat. Res. 6: 345-361 (1972); and the use of Ca.sub.4 (PO.sub.4).sub.2 O was suggested by Brown and Chow in IADR Abstract No. 120, J. Dent. Res. 54: 74 (1975), as a possible pulp capping agent. As described in the latter, Ca.sub.4 (PO.sub.4).sub.2 O hydrolyzes to hydroxyapatite. Such single calcium phosphate cements are incapable of setting to a hard consistency, however, without an additional source of calcium.
Though U.S. Pat. No. 3,913,229 (Driskell et al.) discloses putty-like pastes containing .alpha.-Ca.sub.3 (PO.sub.4).sub.2, .beta.-Ca.sub.3 (PO.sub.4).sub.2, CaHPO.sub.4 and mixtures thereof as pulp capping, root canal, and tooth replanting materials, it is believed that none of these pastes hardens into a cement.
Experience with calcium-based implants for the replacement of skeletal tissue has also existed for many years. Most of these implants have been in the form of prefabricated, sintered hydroxyapatite in either granule or block forms. These preparations have several drawbacks, including a limited ability to conform to skeletal defects, particularly in the case of blocks; inadequate structural integrity of granules (which do not bond together), and difficulty in modeling the implant to the shape of missing skeletal tissue with both blocks and granules. The block form of hydroxyapatite provides structural support, but among other complications, must be held in place by mechanical means, which greatly limits its use and its cosmetic results; and it is very difficult to saw a shape such that it fits the patient's individual defect. The granular form produces cosmetically better results, but has a very limited structural stability and is difficult to contain during and after a surgical procedure. In general, all of these products are ceramics, produced by high temperature sintering, and are not individually crystalline, but rather have their crystal boundaries fused together. These ceramic-type materials are in general functionally biologically non-absorbable (having an absorption rate generally not exceeding on the order of 1% per year).
A porous, non-resorbable material based on coral allows intergrowth with bone, but ultimately becomes only approximately 20% bone with the remaining 80% subsisting as scar tissue. HA RESORB made by Osteogen is a form of absorbable hydroxyapatite, but is not a cement. It is granular and not adhesive. HA RESORB is loosely rather than adhesively packed into place. For large uses, it is replaced by bone too quickly. In the dental materials market, HAPSET is a composition of calcium phosphate granules and cementable plaster of Paris (calcium sulfate). This material is not truly a hydroxyapatite and contains too much calcium sulfate for most biological uses. The calcium sulfate component of such a composition is resorbable, but not the calcium phosphate granules.
In sum, the commercially available hydroxyapatite materials are in general not resorbable with accompanying replacement by bone, and are not self-setting (self-hardening) cements.
The patent literature, does, however, describe at least one class of calcium phosphate cement compositions which are precursors for the formation of hydroxyapatite and are biologically compatible, and have two unique properties that are not attainable in other calcium phosphate biomaterials: (1) self-hardening to form a mass with sufficient strength for many medical and dental applications, and (2) when implanted in bone, the cement resorbs slowly and is completely replaced by new bone formation with no loss in the volume or integrity of the tissue that receives the implant. See U.S. Pat. Nos. Re. 33,221 and Re. 33,161 to Brown and Chow, which teach preparation of calcium phosphate remineralization compositions and of a finely crystalline, non-ceramic, gradually resorbable hydroxyapatite cement based on the same calcium phosphate composition.
A virtually identical calcium phosphate system which consists of tetracalcium phosphate (TTCP) and monocalcium phosphate (MCP) or its monohydrate form (MCPM) was described by Constantz et al. (U.S. Pat. Nos. 5,053,212 and 5,129,905). This cement system is believed to involve conversion of the MCP to dicalcium phosphate which reacts with TTCP and forms hydroxyapatite (HA), the major mineral component of teeth and bone, as the end product.
Constantz et al. U.S. Pat. Nos. 4,880,610 and 5,047,031 describe another cement system that consists of a mixture of solid phosphoric acid crystals, calcium carbonate, and calcium hydroxide as the cement powder and a 7.4 mol/L NaOH solution (4.5 g NaOH in 15 mL of water) as the cement liquid. Data on the physical and chemical properties (compressive strength, hardening time, nature of end product, pH of the cement fluid, heat of mixing etc.) of this cement have not been located in the patent or scientific literature.
The major components of the calcium phosphate remineralizing slurries, pastes and cements taught in U.S. Pat. Nos. Re. 33,221 and Re. 33,161 are preferably tetracalcium phosphate (Ca.sub.4 (PO.sub.4).sub.2 O), and at least one other sparingly soluble calcium phosphate, preferably dicalcium phosphate anhydrous (CaHPO.sub.4), or dicalcium phosphate dihydrate (CaHPO.sub.4.2H.sub.2 O). These react in an aqueous environment to form hydroxyapatite, the principal mineral in teeth and bones, as the final product. Because of the apatitic nature of the set cement, it is highly compatible with soft and hard tissues. This material, if applied intraoperatively as a paste, subsequently sets to a structurally stable implant composed of microporous hydroxyapatitc.